Finishing processes often involve impinging a blast media or an abrasive grit material onto a workpiece to remove dirt, scale, flashing or other such waste. For cleaning small workpieces it is well known in the art to tumble them in a drum within an enclosure which includes a throwing machine for hurling blast materials onto the tumbling pieces. Blast cleaning of larger workpieces or large structures typically involves employing a number of nozzles which direct air entrained abrasive media onto the surface to be cleaned. The workpieces, if manageable, may be moved by conveying means past fixed nozzles. For larger structures, hose-mounted nozzles are moved about as necessary to direct blast media onto the surfaces to be cleaned.
For the smaller workpieces cleaned within an enclosing machine as described above, the blast media, contaminated with removed dirt or waste, falls to the base of the machine from which it is typically augered away to waste or recovery. For the larger cleaning operations, the blast media is likely to be considerably scattered over the work site. The media must be swept up from a relatively large area before conveying it to waste.
When it is desired to recycle a blast media for reuse in a subsequent blast cleaning process, it must be cleaned to remove contaminants and broken media. As shown by Freeman et al. in U.S. Pat. Nos. 3,005,547 and 3,097,452, the recovered media may be subjected to various screening and air separations to remove contaminants. In such cleaning processes, it is conventional to introduce the contaminated material into a rotating drum having apertured cylinder surfaces which allow heavy media of acceptable size to pass through the apertures into a recovery bin. Lightweight contaminating material is blown to waste from the accepted material by causing the material to fall in a thin sheet which is subjected to an air stream. Similar devices that include air separation of lightweight contaminants are described by Freeman in U.S. Pat. No. 3,097,451 and Kriz in U.S. Pat. No. 2,771,189. All these known devices operate on rather finely balanced air separation systems which are difficult to regulate effectively.
In a number of early machines for cleaning grain, lightweight contaminating materials are removed from the grain in a separating drum by sweeping the drum with an air current to carry the fines out, leaving the product grain behind. Robertson in U.S. Pat. No. 1,845,441 includes a separating drum section wherein angular vanes fixed to the interior surfaces of the drum, parallel to the drum axis, lift the grain and tumble it through a counter-current air stream that blows the light material out an elevated end of the drum.
Kahn, in U.S. Pat. No. 3,756,406, describes a pair of rotating concentric coaxial screens, including a first screen which retains straw while passing grain and chaff into an annulus area between the first and second screens. The outer, second screen retains the grain but passes fine dirt. As the grain is conveyed to a discharge end of the apparatus by angled flights mounted on the interior of the larger drum, the grain tumbles and is subjected to a counter-current air stream that blows the dust and chaff out the end of the drum.
The prior patents describe a number of ways of cleaning contaminated material, including apparatus for cleaning contaminated blast media. However, the prior devices all begin with a material that is typically simply dumped into the process or provided by another portion of a manufacturing process. That is, recovery of the media or materials to be cleaned is not an essential part of the recycling process. For example, in the apparatus described by Freeman, noted above, recovery of the blast media is not a problem because the work pieces are relatively small and confined within the apparatus in which the blasting operation is conducted.
As the workpiece to be cleaned become larger and cannot be confined readily, the cleaning operation results in the contaminated blast media being generally scattered about the work site. The beginning point of a recycling operation for those larger cleaning operations is collecting and accumulating the blast media material before cleaning. In most such cases, the collection process requires significant hand labor. These costs, in the past, have simply precluded recycling. Thus, not only has the value of the blast media been lost when it would otherwise be perfectly suitable for cleaning and reuse, but the work site becomes dirty and eventually the materials must be removed to waste anyway. Such waste disposal can today become a problem, particularly where the contaminated blast media after a cleaning operation may include materials regulated as hazardous waste. Disposal cost may be substantial, since contamination with even a small quantity of hazardous waste could require that the entire blast media pile be disposed of as a hazardous waste in a secure waste disposal facility.